It is generally known to dry surface coatings, and especially two component lacquers applied onto metallic or non-metallic components, by means of warm air in a temperature range of e.g. 40 to 90.degree. C., within a convection oven. It is also known to dry such lacquers by means of infrared radiation applied to the lacquer coated articles. Disadvantages common to these two known drying methods are a relatively high energy consumption and a relatively long drying time. Also, in the case of the infrared radiation drying, only components having a relatively simple geometric shape or configuration can be properly dried, because a more complicated geometry would form shadows in the infrared radiation and thus prevent proper uniform drying.
The publication "Deutsche Farben-Zeitschrift" ("German Color Journal") 23rd year, No. 12, (1969), page 585, mentions the possibility of drying lacquers by means of microwave radiation under the heading "Bestrahlung--moderne Trocknungsverfahren fur Lacke". In this context, it was described that commercially available equipment can be used to generate microwaves in the range from 10.sup.8 to 10.sup.10 Hz, which are then directed at the lacquer film on an article so as to heat the film material, which thereby essentially has the effect of transforming the film into the solid state. The heating effect results from the arising dielectric losses, and is highly and immediately effective. Lacquer coatings containing water as a solvent are especially suitable. Such a microwave drying method is also applicable in principle for drying printing inks or dyes. However, it is doubtful whether standard lacquer systems without special dielectric characteristics would actually be dryable in practice by the method proposed in this publication. The described method will be unefficient because the microwaves just heating the thin coating layer which has a small heat capacity due to its low mass. The heat energy of the coating film will rapidly dissipated into the substrate (component) which is not heated up in this microwave process. In particular this will be a disadvantage using thick metall parts.
The English language Abstract of Japanese Patent Publication 4-260472(A), published in the Patent Abstracts of Japan, C-1021, Jan. 28, 1993, Vol. 17, No. 46, describes the use of a special paint coating film that contains a microwave-absorptive self-heat generating powder. In a drying method, such a paint coating film containing the special powder additive is subjected to microwave radiation, which heats the powder and thereby dries or hardens the paint coating film. In this context, the resins or other components of the paint coating film as well as the substrate material must be heat resistant to the necessary degree. Particular characteristics, which the paint coating film must apparently possess for carrying out the mentioned method, are not described in this English language abstract.
French Patent Publication 2,458,323 (Berteaud et al.) describes a method for coating a substrate, by means of which a thin layer of an overcoating is applied onto the substrate. The substrate may especially be made of glass or an analogous material. The overcoating is heat-processed by applying microwave energy thereto. The special feature in this context is that the microwave energy is within a particularly selected frequency range that is adapted for selectively exciting particular components of the overcoating layer. The microwave energy is controlled in such a manner that the frequency of the microwave radiation directed into the drying chamber is varied or adjusted to the extent necessary so that the temperature of the coating is maintained at a constant nominal value. This is achieved using a conventional and commercially available pyrometer. According to this conventional method, the microwaves act directly upon the coating layer. For this purpose, the lacquer or coating materials are partially modified or specially adapted for the microwave drying process, for example by the addition of carbon dust thereto. Only non-conducting substrate materials are used, whereby microwave frequencies higher than 2.45 GHz may also be used.
German Patent Laying Open Document 4,121,203 (Linden et al.) discloses a method and an apparatus for drying a water soluble dispersion lacquer or coating on paper or cardboard webs or sheets by means of microwaves. Preferably, a plurality of commercially available and accepted microwave oscillators are arranged so as to couple the microwave energy into a hollow wave guide. The webs or sheets of paper or cardboard material are dried in a continuous flow-through process, whereby a negligible residual moisture remains in the material, but the method uses a very economical minimum input of energy.